The present invention relates to radiation measuring apparatus and methods. More particularly, the present invention relates to radiation measuring apparatus and methods utilizing a single Geiger Mueller tube (GMT) that provides a count range extending over eight decades, e.g. from background dose rates on the order of 20.times.10.sup.-6 rads (R) per hour (20 .mu.R/h) to high dose rates on the order of 1000 R/h.
Detectors for measuring radiation fields have included ion chambers, proportional counters, Geiger-Muller tubes, scintillation crystals, and solid-state semiconductors. Each has advantages and disadvantages, and the selection of a particular detector depends upon the particular application for which the detector will be used. A single measurement apparatus for measuring radiation fields from very low intensity to extremely high intensity is highly desirable, but has been difficult to achieve due to the limiting physics phenomena of the various detectors at either the high or low regions of radiation dosage.
One of the most common and well-known radiation measuring apparatus is the geiger counter, sometimes called a Geiger-Mueller counter. The geiger counter detects ionizing radiation, including gamma rays and x-rays, and alpha and beta particles. At the heart of a geiger counter is a Geiger Mueller tube (GMT), which typically comprises a glass tube about 2 cm in diameter enclosing a metal cylinder, often of copper, about 10 cm long. (Other dimensions are, of course, also commonly used.) A thin metal wire, e.g. of tungsten, passes along the axis of the metal tube. The cylinder and wire are connected through an end wall of the glass tube to a source of electrical voltage. The tube is filled with a gas, usually a mixture of an inert gas, such as argon or neon, and a halogen, such as chlorine or bromine, at a low pressure, e.g., a few centimeters of mercury. A high voltage, e.g. 550 volts, is set up between the cylinder (which functions as the negative electrode or cathode) and the wire (which functions as the positive electrode or anode). This voltage is just a little less than that needed to create an electrical discharge between the two electrodes.
When a charged particle of sufficient energy enters the GMT, it knocks electrons out of the atoms of the gas. These electrons, being negatively charged, are attracted towards the wire anode, and the atoms from which the electrons originated (which become positively charged ions) are attracted towards the cathode. The high voltage established between the anode and cathode creates a high voltage gradient that accelerates the liberated electrons sufficiently to knock further electrons out of atoms, which in turn are accelerated by the high voltage gradient to knock still further electrons out of other atoms, creating an "avalanche" of electrons. As the avalanche of electrons continues, the positive ions are also accelerated towards the cathode wall. These positive ions strike the cathode wall with sufficient energy to release still additional electrons. All of these electrons descend on the anode wire and are detected as a pulse of electric current. The occurrence of this pulse thus indicates that a charged particle has passed through the tube. The electrical pulses can then be amplified and counted, using appropriate electronic counting circuitry, and/or converted to audible sound, to provide a user of the geiger counter a quantitative and/or qualitative measure of the number of charged particles encountered by the GMT.
Unfortunately, the rate at which charged particles can be detected by the GMT is limited. This is because during a discharge, i.e., during that time during which the electron avalanche is occurring, the GMT is insensitive to further charged particles arriving at the detector. Thus, some means must be employed to stop the electron avalanche, and to prepare the GMT to detect the next arriving charged particle.
One common technique used to help stop the avalanche is to reduce the voltage potential between the anode and cathode. Some reduction of this voltage occurs naturally as the electrical pulse developed on the anode effectively discharges the charged GMT (which may be considered prior to discharge as a charged capacitor). However, it is also known in the art to deliberately decrease the applied voltage potential for a sufficient time to allow the electron avalanche to sweep out of the GMT, at which time the voltage is again raised to a value just a little less than needed to create an electrical discharge. See, e.g., U.S. Pat. Nos. 4,605,859 and 4,631,411.
The time it takes to stop the electron avalanche and prepare the GMT to detect the next charged particle limits the rate at which charged particles may be detected. Most conventional GMT counters have a recovery time on the order of 10.sup.-5 seconds, or 10 .mu.sec. In other words, after detecting a charged particle (as evidenced by the occurrence of an electrical pulse), it takes at least 10 .mu.sec for the charge to be swept out of the tube before the tube can be readied to detect the next charged particle. Thus, such conventional geiger counters are able to detect charged particles or radiation at a rate that is limited to no more than about 100,000 pulses per second. Unfortunately, where the sensitivity of the GMT must be on the order of 25 cps/(mR/H), as indicated below, this recovery time significantly limits the high dosage rate at which radiation can be detected.
In addition to being rate limited, it is common in the art to employ differently constructed GMTs to detect different levels of radiation. That is, a particular type of gas, tube size, tube materials, and applied anode voltage may be better suited for detecting low levels of radiation than is best suited for detecting high levels of radiation. Hence, depending upon the particular level of radiation expected, a different GMT may be needed. Thus, in order to detect a wide range of radiation levels for those applications where use of a GMT is indicated, it has frequently heretofore been necessary to maintain an inventory of several GMT-based measuring devices.
The ideal radiation measurement device would be portable and would typically include operation from background radiation levels on the order of 20 .mu.R/h to high radiation levels on the order of 1000 R/h. Accuracy and response time requirements at the lower end of this range have typically dictated the use of a detector with a sensitivity of at least 25 cps (counts per second) per mR/h. Unfortunately, the photon interaction rate in a 25 cps/(mR/h) GMT is around 2.5.times.10.sup.-7 sec.sup.-1 at the upper dose rate limit of 1000 R/h. Hence, the relatively long resolving times of GMT's (tens of microseconds) requires that the counting circuit be more than just a fast counter.
Radiation measurement instruments are available, see U.S. Pat. Nos. 4,605,859 and 4,631,411, that address this problem by lowering the anode voltage below the Geiger threshold until the tube is in a fully recovered condition (i.e., until all space charge from the previous Geiger events has been swept from the tube), and then quickly increasing the voltage above the threshold, and then measuring the time interval to the next Geiger pulse. This is essentially a reciprocal rate measurement (i.e., dosage rate is determined by measuring the period of the time between raising the anode voltage to its operating level and receiving the next Geiger pulse), and the statistical accuracy of such measurement is improved by averaging successive measurements.
The reciprocal rate measurement technique can only be as accurate as the starting point for measuring the Geiger period. Unfortunately, this starting point, when the anode voltage is precisely at its proper operating value, is not known with a great deal of certainty. Hence, as the Geiger period becomes shorter and shorter (higher and higher dosage rates), the uncertainty of the starting point of the period contributes a larger and larger error to the measurement. Because the measurement thus becomes increasingly inaccurate for short time intervals, reciprocal rate measurement instruments of the prior art have hereto employed a relatively sensitive GMT to span the lower part of the dynamic range and a less sensitive GMT to span the upper decades. Unfortunately, this approach requires separate calibration factors for each tube and provisions in both the hardware and firmware for selecting only one of the two tubes. What is needed is measurement apparatus that utilizes the advantages of a reciprocal rate measurement, yet only requires a single GMT (thereby simplifying both hardware and firmware/software), and that eliminates the uncertainties of the beginning of the Geiger period, thereby allowing statistical techniques to be used to make an accurate estimate of the high dosage rates. The present invention advantageously addresses these and other needs.